The invention is directed to a noise cancelling telephone transmitter for use in a telephone handset. The transmitter comprises a cap and a cup removably coupled to the cap so as to form an assembly insertable as a unit in the transmitter receptacle of the handset. A transmitter of this type is generally described in co-pending U.S. patent application Ser. No. 366,501 assigned to the assignee herein. The application discloses an anti-side tone transmitter comprising a cap and cup removably coupled together. The cap is provided with a central opening and a series of satellite openings surrounding the central opening. An electret microphone is disposed below the cap such that the acoustic entrance of the microphone is substantially flush with the underside of the cap at the region of the central opening. A depending annular wall serves as an acoustic baffle between the cap satellite openings and the microphone acoustic entrance. An acoustic damping element, in the form of a large foam disk, surrounds the microphone below the satellite openings. Sound passing through the satellite openings is damped by the foam disk. Sound passing through the central opening enters the microphone acoustic entrance and is converted to electrical signals by a transmitter circuit connected to the microphone.
Heretofore, a transmitter of the type described above has not been able to provide satisfactory voice frequency and noise cancelling responses at the same time in the environment of a conventional telephone handset such as the Western Electric G-type of K-type handsets. Thus, it is desirable for a telephone transmitter to provide substantially flat voice and noise frequency responses from approximately 20 hz to 5 Khz with maximal signal to noise ratio. Various designs have been attempted to optimize the voice frequency and noise responses wherein satellite openings are provided in the cap and cup. In general, these designs have been unsatisfactory in achieving the desired voice frequency response while at the same time obtaining the desired noise cancelling effect. There is, accordingly, a need for such a telephone transmitter which provides both optimal voice frequency response and noise cancelling effect.
An object of the invention is to provide a telephone transmitter of the type which is insertable as a unit in the receptacle of a conventional telephone handset and which provides optimal voice frequency response and optimal noise cancelling effect.
Another object of the invention is to provide a telephone transmitter which may be substituted easily for the conventional carbon granule telephone transmitter.